This means that the gem sunlight is not installed or at least not
installed correctly.

I installed sunlight on my system and it works fine:

$ sudo gem install sunlight
Password:
Successfully installed ym4r-0.6.1
Successfully installed sunlight-1.1.0
2 gems installed
Installing ri documentation for ym4r-0.6.1...
Installing ri documentation for sunlight-1.1.0...
Installing RDoc documentation for ym4r-0.6.1...
Installing RDoc documentation for sunlight-1.1.0...
$ irb
irb(main):001:0> require 'sunlight'
=> true
irb(main):002:0> ^D
$